To cerbusjf: Thank you so much for your suggestions. Here are three photos taken the other day at the marina. The first one focused as it should. However, the second one was taken later and that photo also was o.k. Later I tried to take another of the same boat and that's when the shutter would not activate. Just before going home I took another photo of a nearby boat and the shutter activated once again.

I was using aperture mode, single focus and single area mode. As you can see from the images it was a beautiful day. This problem seems to only happen when I zoom in and not when zoomed out.

This was the 3rd time that this happened, once with 55-200mm and the other 2 times with the 18-108mm lens.
Shirley